Fix VSTHRD107 to detect unawaited tasks in using variable declarations

[Austin (AustinAviv)]

Fixes #1697

Description

This PR expands VSTHRD107AwaitTaskWithinUsingExpressionAnalyzer and VSTHRD107AwaitTaskWithinUsingExpressionCodeFix to detect and fix unawaited Task / Task expressions when used within using variable declarations:

• using (var releaser = semaphore.EnterAsync())
• using (IDisposable releaser = semaphore.EnterAsync())
• using var releaser = semaphore.EnterAsync(); (C# 8 using declarations)

Background

Because Task implements IDisposable, storing an unawaited task in a using variable declaration compiles without warning, but disposes the unawaited task instance instead of awaiting the releaser.

Previously, VSTHRD107 only inspected UsingStatementSyntax.Expression. It missed declaration forms, and VSTHRD107AwaitTaskWithinUsingExpressionCodeFix threw an exception when .Expression was null.

Changes

1. Analyzer:
   • Registered SyntaxKind.LocalDeclarationStatement in addition to SyntaxKind.UsingStatement.
   • Added checking for usingStatement.Declaration and localDeclaration.Declaration variable initializers.
   • Handled C# 8 using var declarations (ignoring wait using).
2. Code Fix:
   • Replaced direct mutation of usingStatement.Expression with node replacement targeting the containing statement (ReplaceExpressionWithAwait).
   • Cleanly wraps expressions with wait without unnecessary parentheses around invocations or identifier names.
3. Docs and Tests:
   • Updated documentation in docfx/analyzers/VSTHRD107.md.
   • Added unit tests covering sync and async methods, using (var ...), using var ...;, and explicit IDisposable type annotations.

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🟡 Changes recommended

Some offered fixes produce uncompilable code for unsupported containing functions or tasks without disposable results.

1 open finding

• Make the nearest await-capable function async · New

What changed in this PR

Expands VSTHRD107 to detect unawaited tasks in using declarations and updates its code fix.

Changes:

• Analyzes using-statement and C# 8 using-declaration initializers.
• Reworks code-fix expression replacement.
• Adds documentation and test coverage.

File	Description
VSTHRD107AwaitTaskWithinUsingExpressionAnalyzerTests.cs	Tests new declaration forms.
VSTHRD107AwaitTaskWithinUsingExpressionAnalyzer.cs	Analyzes using-variable initializers.
VSTHRD107AwaitTaskWithinUsingExpressionCodeFix.cs	Adds await to diagnosed expressions.
VSTHRD107.md	Documents supported forms.

🧠 Review effort: Balanced

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[Austin (AustinAviv)]

Hi, could any maintainer please review and approve this PR when you get a chance? I'd like to check the workflow test results to see how many tests passed and failed. Thank you!

[Austin (AustinAviv)]

Hi maintainers, could you please approve the workflow run for this PR? Thank you!

[Drew Noakes (drewnoakes)]

Austin (@AustinAviv) I approved the workflows to run. I'll defer to Andrew Arnott (@AArnott) for the actual merge.

[Austin (AustinAviv)]

Austin (Austin (@AustinAviv)) I approved the workflows to run. I'll defer to Andrew Arnott (Andrew Arnott (@AArnott)) for the actual merge.

Hi Andrew Arnott (@AArnott) Drew Noakes (@drewnoakes), thanks, Drew, for approving the previous workflow run! Could you please approve the new workflow run as well if approval is required? Andrew, when you get a chance, I'd appreciate your review of this PR. I'm happy to address any feedback or make further changes. Thanks both for your time and support!